Ion exchange membranes are used as an ion separation membrane in electrodialysis and diffusion dialysis for a wide variety of applications such as condensation of seawater, desalting and removal of nitrate-nitrogen from underground brine for providing drinking water, desalting in a process for producing a food and condensation of an active ingredient for a medicinal drug. Ion exchange membranes useful in these applications are typically styrene-divinylbenzene based homogeneous ion exchange membranes, which have been improved to such a level that they can be employed in an industrially useful separation after development of various technologies such as permselectivity of monovalent and divalent ions, improvement in selectivity for a particular ion and lowering a membrane resistance.
Generally, salts are often formed as by-products in a process for producing an organic compound in a field such as foods, medicinal drugs and pesticides. Salts contained in such an organic compound are often separated by electrodialysis. In separation of salts by electrodialysis, a direct current is applied to cation exchange membranes and anion exchange membranes which are alternately aligned, to transfer cations to a cathode side of the cation exchange membrane and to transfer anions to an anode side of the anion exchange membrane, respectively. Thus, desalting is achieved by removing salts from an electrolyte solution in a room sandwiched by the cathode side of the cation exchange membrane and the anode side of the anion exchange membrane. Electrodialytic desalting of a liquid to be processed is accompanied by a problem of so-called organic fouling of a membrane where membrane performance is deteriorated due to adhesion of an organic contaminant, particularly electrically-charged macromolecules (hereinafter, referred to as “macro-organic ions”) in a liquid to be processed.
In an anion exchange membrane, which is particularly susceptible to organic fouling, membrane performance is gradually deteriorated as a dialysis cycle proceeds, and in some cases, with marked contamination, a membrane swollen or broken in a relatively shorter time.
An anion exchange membrane preventing organic fouling which is well-known in the art is an anion exchange membrane which is designed to prevent macro-organic ions from permeating into the membrane. This membrane has a structure where a thin film which is neutral, amphoteric or oppositely charged to an ion exchange group is formed on the surface of the membrane. The more compact a membrane structure is and the larger a molecular weight of the macro-organic ion, the more effective such an anion exchange membrane is. For example, there has been described an anion exchange membrane wherein the surface is modified with a sulfonic group having an opposite charge for preventing an anionic macro-organic ion from permeating into the membrane (Patent Reference 1). Furthermore, there has been an anion exchange membrane in which resistance to organic fouling is improved by optimizing a counter-ion structure in an anion exchange group (Patent Reference 2).